diff --git a/src/querytyper/mongo/query.py b/src/querytyper/mongo/query.py
index 70a6130..6f52945 100644
--- a/src/querytyper/mongo/query.py
+++ b/src/querytyper/mongo/query.py
@@ -172,7 +172,13 @@ def __contains__(
         other: T,
     ) -> QueryCondition:
         """Overload in operator."""
-        return regex_query(self.name, re.compile(other))
+        if not issubclass(cast(type, self.field_type), str):
+            raise TypeError(
+                f"Cannot check if field {self.name} contains {other} because {self.name} is not a subclass of str but {self.field_type}"
+            )
+        if not isinstance(other, str):
+            raise ValueError("Comparison value must be a valid string.")
+        return self == {"$regex": other}
 
 
 def exists(
diff --git a/tests/mongo/test_pymongo_integration.py b/tests/mongo/test_pymongo_integration.py
index 01ae691..898512e 100644
--- a/tests/mongo/test_pymongo_integration.py
+++ b/tests/mongo/test_pymongo_integration.py
@@ -30,6 +30,6 @@ def test_integration_with_pymongo() -> None:
     assert found_dummy.int_field == 1
     query = MongoQuery("test" in QueryModel.str_field)
     assert isinstance(query, dict)
-    # assert query
+    assert query
     found_docs = list(collection.find(query))
     assert len(found_docs) == doc_num